1. Field of the Invention
The object of the present invention is a method for sealing vacuum tube tips (also known as stems), especially the tips of cathode tubes, and a machine for applying this method.
2. Description of the Prior Art
To seal the evacuation tips of vacuum tubes, first an adequate vacuum is made in the tube, then this tip is heated to melting point by means of a heating resistor that surrounds it at a small distance away, set in a refractory chamber forming a sort of furnace around this tip. The heating operations generally comprise three main stages: pre-heating, melting and annealing. These stages are executed at different successive stations along the path followed by the trolleys that carry the tubes and the device that heats their tips after these tubes are removed. At each station where the trolleys stop, the heating devices of these trolleys receive a certain amount of electrical power given by a generator through a set of sliding contacts. The value of this power depends on the temperature reached by the tips, this temperature being determined beforehand, according to a established process in which a standard quartz tip is used.
This method used in the prior art has two main disadvantages: because of the variations in the value of the resistance of the sliding contacts and the resistance of the current lead-in wires for the various heat treatment stations, as well as variations in the heating resistor itself because of its ageing, it is not possible to guarantee the requisite heating temperature for each station, and this creates the risk of breakage or embrittlement of the tip after sealing, owing to the poor coordination of the heating temperatures at the various stations. Furthermore, if for any reason, the trolleys stop for longer than planned at their respective stations, there could be a loss of synchronization between the real position of the trolleys and the heat treatment stages, leading to loss of control over the pre-set process.
3. Summary of the Invention
The object of the present invention is a method to seal evacuation tips, a method that guarantees the desired heating temperature at each station even when the trolleys stop for longer than is necessary at a treatment station.
The method for sealing evacuation tips according to the invention consists in regulating the heating power sent to the tip-heating device for each heating stage, at a constant set value.
The device according to the invention, inserted in a vacuum tube production line, especially a cathode tube production line, downstream of the machines that evacuate the tubes and treat them before they are sealed, comprises an energy supply device that works, together with tube-carrying trolleys fitted with appropriate tip-heating devices, at several treatment stations, for example by means of current distribution rails. This energy supply device comprises a regulated power supply source, controlled by a computer device that lays down a pre-determined set power value on each station.
According to an advantageous feature of the invention, the computer device works together with means to detect any prolonged stopping of the tubes at treatment stations and means that change the set power value at any station where the stopping time of a tube has exceeded a determined value to the following set value.